1. Field of the Invention
The present invention relates to a micro-oscillation element such as a micro-mirror element with a movable portion for which rotary displacement is possible.
2. Description of the Related Art
Recently the application of infinitesimal devices created by micro-machining technology are being attempted in various technical fields. In the field of optical communication technology, for example, micro-mirror elements with light reflecting functions are receiving attention.
In optical communications, an optical signal is transmitted by using an optical fiber as a medium, and furthermore, in general, an optical switching device is used in order to switch the transmission path of the optical signal from one fiber to another fiber. Characteristics required in an optical switching device in order to achieve good optical communications include high capacity, high speed and high reliability in the switching operation. From this point of view, expectations have been growing with regard to optical switching devices which incorporate micro-mirror elements fabricated by micro-machining technology. This is because the use of a micro-mirror element makes it possible to carry out switching processes on the optical signal itself, without having to convert the optical signal to an electrical signal, between the optical transmission path on the input side of the optical switching device and the optical transmission path on the output side thereof, and this means that it is suitable for obtaining the above-described characteristics.
A micro-mirror element is provided with a mirror surface for reflecting light, and it is capable of changing the direction of light reflection by oscillation of the mirror surface. Electrostatic drive-type micro-mirror elements which use electrostatic force in order to cause the mirror surface to oscillate are used in many devices. Electrostatic drive-type micro-mirror elements can be divided broadly into two types: micro-mirror elements manufactured by so-called surface micro-machining technology, and micro-mirror elements manufactured by so-called bulk micro-machining technology.
In the case of surface micro-machining technology, a thin layer of material corresponding to a respective constituent area is formed on a substrate and processed into a prescribed pattern, and such patterns are layered in a sequential fashion, whereby respective areas constituting an element, such as a support, an oscillating portion, a mirror surface and an electrode section, are formed. In addition to these portions, a sacrificial layer, which is subsequently removed, is also formed. On the other hand, in the case of bulk micro-machining technology, a support and an oscillating portion are formed in a prescribed shape by etching the material substrate. Thereafter, a mirror surface and an electrode is formed by a thin-layer forming process. Bulk micro-machining technology is described, for example, in Japanese Patent Laid-Open No. (Hei)10-190007, Japanese Patent Laid-Open No. (Hei)10-270714 and Japanese Patent Laid-Open No. 2000-31502.
One technical feature required in a micro-mirror element is that the mirror surface which performs light reflection has a high degree of flatness. However, in the case of surface micro-machining technology, since the mirror surface ultimately formed is thin, the mirror surface is liable to curve, and consequently, it is difficult to achieve a high degree of flatness in a mirror surface having a large surface area. On the other hand, in the case of bulk micro-machining technology, a mirror section is constituted by cutting into the material substrate, which is relatively thick, by means of an etching process, and since a mirror surface is provided on this mirror section, it is possible to ensure rigidity, even if the mirror surface has a large surface area. Consequently, it is possible to form a mirror surface having a sufficiently high degree of optical flatness.
FIGS. 20–21 illustrate a conventional electrostatically driven micro-mirror element X5 fabricated by the bulk micro-machining technology. FIG. 20 is an exploded view showing the micro-mirror element X5, while FIG. 21 is a cross-sectional view along line XXI—XXI in FIG. 20 of the micro-mirror element X5 in the assembled state.
The micro-mirror element X5 has a structure in which a mirror substrate 200 and a base substrate 206 are layered on each other. The mirror substrate 200 is constituted by a mirror supporting section 201, a frame 202, and a pair of torsion bars 203 linking the section 201 and the frame 202. By performing etching from one side of a material substrate, such as a silicon substrate having electrical conductivity, it is possible to form the outline shape of the mirror supporting section 201, frame 202 and torsion bars 203 on the mirror substrate 200. A mirror surface 204 is provided on the upper surface of the mirror supporting section 201. A pair of electrodes 205a, 205b are provided on the lower surface of the mirror supporting section 201. The pair of torsion bars 203 defines a rotational axis A5 for the rotational operation of the mirror supporting section 201. The base substrate 206 is provided with two electrodes 207a and 207b which oppose the electrodes 205a and 205b of the mirror supporting section 201, respectively.
In the micro-mirror element X5, when an electric potential is applied to the frame 202 of the mirror substrate 200, the electric potential is transmitted to the electrodes 205a and 205b, through the torsion bars 203 and the mirror supporting section 201, which are formed integral with the frame 202 from the same conductive material. Consequently, by applying a prescribed electric potential to the frame 202, it is possible to charge the electrodes 205a and 205b, positively, for example. In this state, if the electrode 207a of the base substrate 206 is charged with a negative charge, then an electrostatic attraction is generated between the electrode 205a and the electrode 207a, and hence the mirror supporting section 201 rotates in the direction of the arrow M5, as indicated in FIG. 21, whilst twisting the torsion bars 203. The mirror supporting section 201 is able to swing until it reaches an angle at which the force of attraction between the electrodes balances with the twisting resistance of the torsion bars 203. Alternatively, if a negative charge is applied to the electrode 207b whilst a positive charge is applied to the electrodes 205a, 205b of the mirror supporting section 201, then an electrostatic attraction is generated between the electrode 205b and the electrode 207b, and hence the mirror supporting section 201 will rotate in the opposite direction to the arrow M5. By driving the mirror supporting section 201 to swing as described above, it is possible to switch the direction of light reflected by the mirror surface 204.
In order to decrease the size of the micro-mirror element X5 in the longitudinal direction of the axis A5, it is necessary to make smaller the length L51 (see FIG. 20) of the mirror supporting section 201, or the length L52 of the frame 202, or the length L53 of the torsion bars 203. However, as the length L51 of the mirror-supporting section 201 becomes smaller, the area of the mirror surface 204 formed on the upper surface of the mirror supporting section 201 becomes smaller. Accordingly, it becomes difficult to obtain appropriate light-reflecting performance for the switching device. In addition, the reduction of the length L51 of the supporting section 201 leads to the reduction of the areas of the electrodes 205a, 205b formed on the lower surface of the supporting section 201. As the electrodes 205a, 205b have a smaller area, it is difficult to attain reduction of the driving voltage needed for operating the switching device. Turning to the frame 202, the length L52 should not be too small for giving required rigidity to the frame 202. Likewise, the length L53 of the torsion bars 203 should not be too small for ensuring appropriate mechanical properties (spring constant, strength, etc.) of the torsion bars.
As described above, the conventional micro-mirror element X5 has a structure with which the size reduction in the longitudinal direction of the axis A5 is difficult. Generally speaking, a micro-mirror element is required to provide a large rotational angle and high rotational speed, with low driving voltage. The conventional mirror element X5 cannot meet these requirements when it is reduced in size.